This invention relates to apparatus for driving displays. This apparatus is particularly but not exclusively intended for driving electrophoretic displays, especially colored electrophoretic displays capable of rendering more than two colors using a single layer of electrophoretic material comprising a plurality of colored particles. The term color as used herein includes black and white.
The term gray state is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, several of the E Ink patents and published applications referred to below describe electrophoretic displays in which the extreme states are white and deep blue, so that an intermediate gray state would actually be pale blue. Indeed, as already mentioned, the change in optical state may not be a color change at all. The terms black and white may be used hereinafter to refer to the two extreme optical states of a display, and should be understood as normally including extreme optical states which are not strictly black and white, for example the aforementioned white and dark blue states.
The terms bistable and bistability are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in U.S. Pat. No. 7,170,670 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called multi-stable rather than bistable, although for convenience the term bistable may be used herein to cover both bistable and multi-stable displays.
The term impulse, when used to refer to driving an electrophoretic display, is used herein to refer to the integral of the applied voltage with respect to time during the period in which the display is driven.
A particle that absorbs, scatters, or reflects light, either in a broad band or at selected wavelengths, is referred to herein as a colored or pigment particle. Various materials other than pigments (in the strict sense of that term as meaning insoluble colored materials) that absorb or reflect light, such as dyes or photonic crystals, etc., may also be used in the electrophoretic media and displays of the present invention.
Most commercial electrophoretic displays are monochrome, typically black and white. However, attempts have recently been made to develop electrophoretic displays which can display more than two colors, and preferably as many as eight colors, at each pixel. See, for example, U.S. Pat. Nos. 8,717,664 and 9,170,468; and US 2014/0313566; US 2014/0340734; US 2014/0340736; and US 2015/0103394; and the aforementioned US 2014/0340430 and US 2016/0085132. Many of these colored electrophoretic displays require the use of more than three voltage levels to drive the display; various displays described in the applications specifically mentioned above require five or seven voltage levels. Some of the aforementioned displays also make use of active matrix displays with front plane switching, in which the voltage on the common front electrode is varied during the driving process. This is in contrast to most prior art monochrome displays which only require the use of three voltage levels, typically −V, 0 and +V, where V is the drive voltage. Because most commercial monochrome displays only require the use of three voltage levels, typically the column (data line) drivers available for use with such displays are only arranged to handle three voltage levels at any one time (i.e., in any one scanning period (frame period) of the display). To avoid the delay and expense of developing custom drivers for colored displays, it is highly desirable to be able to use the commercial three level drivers to drive colored displays. As described in the aforementioned US 2016/0085132, it is possible to operate a display requiring the use of five, seven or more voltage levels using a driver capable of handling only three voltage levels in any one frame period by careful arrangement of the waveforms to be used in the display, but to do so it is necessary to be able to change the voltages available from the three level driver on a frame-by-frame basis. Although apparatus capable of changing voltages on a frame-by-frame basis can be assembled from conventional electronic control devices, such apparatus would be inconveniently bulky and costly for use with a small electrophoretic display, for example an electronic book (or document) reader, and hence there is a need for compact, inexpensive apparatus for this purpose. The present invention seeks to provide such apparatus.